A variety of different signal detectors have been developed in the past, and, indeed, many electronic circuits include tuning circuitry adapted to receive, filter and process signals within a selected frequency range. With respect to signal detectors constructed specifically to monitor a broad range of oscillatory signals, certain problems arise where those circuits seek signals having certain selected frequency characteristics. For example, many of the existing detectors exhibit a high susceptibility to noise and produce faulty readings where random noise signals fall within the target frequency range. Further, these systems often encounter problems with signal to noise ratio for the target signals versus the background noise. In order to solve these problems, many existing signal detectors resort to superheterodyning in order to increase sensitivity, eliminate noise and shift frequency bands.
As noted above, the present invention especially concerns the detection of ultrasonic signals in an ambient sound environment. The desirability of these devices has been recently increasing due to the recognition that ultrasonic detectors may readily be implemented as leak detectors to detect ultrasonic signals which, for example, are created by the escape of pressurized gasses through small openings. This is useful, for example, in detecting leakage from pipelines as well as in detecting air flow paths, for example, through insulation of houses and commercial buildings and through automobile doors and panels. Other analytical values of such ultrasonic detectors are being discovered as well.
Two types of ultrasonic detectors currently dominate the market. A first type employs a crystal system to mechanically couple an ultrasonic input signal to a local oscillator in order to convert the frequency of the input ultrasonic signal to a resultant signal that has a frequency within the audible range. While being relatively inexpensive, crystal-based systems exhibit limited performance and have significant problems of sensitivity. These crystal-based systems are susceptible to noise, have problems with signal to noise ratio. In addition, though, crystal based systems are susceptible to mechanical vibrations and are susceptible to temperature changes which can effect their sensitivity and yield false readings. Further, crystal-based systems often and undesirably respond to infra-sonic and sonic signals that modulate the system so that again faulty readings occur. These crystal-based systems further usually have a very limited frequency range for target signals unless there is an ability to adjust the frequency of the local oscillator within the system.
A second system commonly used employs signal mixers that heterodyne a local oscillator with the input signal. Again, these systems are susceptible to noise, have problem with signal to noise ratio and have a limited frequency range unless the oscillator frequency can be adjusted. While these systems do not exhibit problems due to sonic or mechanical vibrations, they are nonetheless susceptible to temperature changes that can yield faulty readings. Further, systems that employ the heterodyne technique require multi-offset settings and are thus difficult to adjust and maintain over an extended period of use.
Accordingly, there remains a long felt need for a signal detector that is both sensitive in operation and which can operate to detect the large range of target frequencies. Further, there is a need for a signal detector and methodology which is less susceptible to mechanical vibration and temperature change. There is a further need for signal detectors and methodology that are more effective at filtering noise signals in order to get a cleaner target signal detection.